Concurrency based on promises

A promise is a value that may become determined in the future.

Lwt provides typed, composable promises. Promises that are resolved by I/O are
resolved by Lwt in parallel.

Meanwhile, OCaml code, including code creating and waiting on promises, runs in
a single thread by default. This reduces the need for locks or other
synchronization primitives. Code can be run in parallel on an opt-in basis.